In order to observe the appearance of the surface of a sample, the surface can be viewed from a certain direction, whereby a light beam is directed to the surface from another direction. Thereby, a variety of information about the surface can be obtained, depending on the direction, intensity and color of the incoming and outgoing light and on the direction of viewing towards the surface. The observed appearance may include the texture and/or relief or protrusions or projections of the surface such as hair on the skin and, in case the surface itself is more or less translucent, the texture and/or color and/or morphology underneath the surface, i.e. the sub-surface. The observation can be recorded and/or analyzed.
It should be noted that the expression appearance is used in this description for each combination of aspects and/or properties of the surface of the sample and the perception of the observer of it, including the relief of the surface, the color of the surface, the light reflecting and light absorbing properties of the surface, etc. Observing is a general expression, it may include inspecting and/or recording and/or analyzing of the appearance of the surface.
A non-contact and non-perturbing monitoring technique is useful in many areas of technology to determine surface and/or sub-surface morphology. Furthermore, the type and density of material defects or other features, which have a geometric shape, can be characterized using this technique. Another use of this technique is the analysis of the characteristics and condition of human skin.
In particular when details of the morphology are to be analyzed, it is desired to make observations of the appearance of the surface from different directions, whereby the light source also may illuminate the surface from one or more predetermined directions (different angles with respect to the plane of the surface).
In particular when the surface of a relative large object has to be observed, for example a piece of the skin of a human body, it is not possible to place the sample inside the apparatus. In that case, the apparatus should be placed on or against the sample or a part of the sample, whereby the location of the surface to be observed is at the outer side of the apparatus.
An optical measurement device for measuring an optical appearance of a surface of a sample is disclosed in WO 2007/072403. The disclosed device comprises an illumination device for illuminating the surface with an illumination beam and a detection device for detecting the response of the sample to the illumination beam.
In the case that the sample is of a semitransparent material, the light from the light source not only reflects on the surface of the sample, but also penetrates in the sample. The penetrated light scatters inside the sample and will at least be partially absorbed inside the sample, which is characterized by a so-called extinction length, which is a measure for the distance over which all of the penetrated light will be absorbed. Hence, only a part of the penetrated light will be absorbed in the case that the size of the illuminated region is relatively small compared to the extinction length of the penetrated light. If not all of the penetrated light is absorbed, a part of the penetrated and scattered light will emit from the surface of the sample. The observance of the reflected light is then disturbed by the scattered light that emits from the surface of the sample. Therefore, the accuracy of the observation of the appearance of the sample will be relatively poor in the case that the size of the illuminated region is relatively small compared to the extinction length of the penetrated light.